The present invention relates to a device for detecting the amount of intake air for by an internal combustion engine and, more particularly, to an apparatus for mounting an intake air flow rate detector in an intake air path of an internal combustion engine.
A fuel injection system of an internal combustion engine must continuously measure the amount of the intaken air. This measurement is taken by a vortex flow meter in which a vortex generator is placed in an intake air path, and Karman vortices which results on the downstream side of the vortex generator are measured to determine the flow rate of the fluid. This vortex flow meter contains no moving parts and is not effected by vibration, lending itself well for use in an internal combustion engine which vibrates and, particularly, for use in an automobile.
FIG. 1 shows a conventional vortex flow meter that is disclosed in Japanese Patent Publication No. 56415/1983, wherein reference numeral 1 denotes a flow meter having a vortex generator 2, and 3 denotes the Karman's vortex street that is generated. The vortex generator 2 is installed in an intake air path of an internal combustion engine. Reference numeral 4 denotes an ultrasonic wave generator which is provided so as to propagate ultrasonic waves across the Karman's vortex street 3, reference numeral 5 denotes an ultrasonic wave receiver which receives ultrasonic waves, numeral 6 denotes an oscillation circuit for oscillating the ultrasonic wave generator 4, reference numeral 7 denotes a voltage controlled phase deviation circuit which controls the phase deviation angle from the output signal of the oscillation circuit 6 dependng upon the output voltage of a loop filter 10, reference numeral 8 denotes a first waveform shaping circuit which amplifies and shapes the output of the ultrasonic wave receiver 5, reference numeral 9 denotes a phase comparator which receives, as a first input signal, the output of the first waveform shaping circuit 8 and receives, as a second input signal, the output of the voltage controlled phase deviation circuit 7, in order to detect the phase difference between the first input signal and the second input signal, numeral 10 denotes a loop filter which removes unnecessary frequency components from the output of the phase comparator 9, and 11 denotes a low-pass filter which removes carrier frequency components from the output of the phase comparator 9.
The operation of the prior art device will now be described. First, ultrasonic waves generated from the ultrasonic wave generator 4 are phase-modulated by the Karman vortices 3, and are received by the ultrasonic wave receiver 5. The received waves are then shaped by the waveform shaping circuit 8. A phase synchronizing loop is constituted by the oscillation circuit 6, voltage controlled phase deviation circuit 7, phase comparator 9 and loop filter 10. The voltage controlled phase deviation circuit 7 controls the phase deviation angle to maintain only a high frequency stability of the signals of an ultrasonic wave oscillation frequency. Further, characteristics of the loop filter 10 in the phase synchronizing loop are set to be capable of following at a sufficiently high speed the modulated angular frequency of the signals of which the phase is modulated by Karman vortices. The output of the loop filter 10 so changes the output of the voltage controlled phase deviation circuit 7 that it is brought into synchronism with the ultrasonic received signal, and is directly used as a phase demodulated output. A phase synchronizing angle of the phase synchronizing loop is determined by the characteristics of the phase comparator 9 and of the loop filter 10.
The intake air of an internal combustion engine does not flow uniformly but pulsates. Therefore, the Karman's vortex street 3 generated by the vortex generator 2 is unstable. Pulsation of the intake air develops particularly in a multi-cylinder engine in which the intake valves operate in an overlapped manner. The overlapping gives rise to the formation of a surge pulse of negative pressure at a moment when the intake valve is opened. When the throttle valve is nearly fully opened, there is obtained a no orifice effect and the surge pulse of a negative pressure does not attenuate. This fact causes Karman's vortex street 3 to develop irregularly. In the intake air path, therefore, the pressure varies as indicated by a dotted line in FIG. 2(a). Hence, the output of the vortex flow meter at the low-pass filter 11 pulsates as indicated by a solid line in FIG. 2(a). FIG. 2(b) shows this output which shaped as a step function, wherein the broken lines indicate that outputs are missing, improperly detecting the number of Karman vortexes 3, and, thus amount of the air intaken by the internal combustion engine is not correctly dectected by the vortex flow meter. In copending Application Serial Number 07/038,607, filed Apr. 15, 1987, a compensating system is disclosed which is added to a vortex flow meter for removing pressure-dependent components from the output due to fluctuations in pressure in the intake air stream. This compensation system utilizes a pressure sensor which senses pressure and produces an output representing the pressure fluctuations.